The use of Flats

[leon]

Hi Guys, please correct me if i'm wrong, but i suspect that if one has taken flats and combined them for a master flat he/she could use that master flat for all the images he/she takes through the instrument that has taken the lights only if nothing has been altered in the optical path of the imaging device.

Gee that was a mouth full, but would this be correct.

Leon

[Dennis]

That sounds right on the money Leon. No changes to the camera orientation allowed. No changes to the camera focus allowed.

If you change camera orientation, then any dust mote shadows, etc would be recorded at different X-Y pixel locations on the CCD chip, thus not matching the original non-rotated positions.

If you change the focus, then the shadows caused by the dust motes would change in size and not match the original.

Cheers

Dennis

[[1ponders] (Paul)]

That is correct Leon. You can create a library of master darks if you want to take the time for it but your flats need to be taken prior to any alterations to your imaging setup, even focusing. So if you are going to rotate your camera you should take flats before you rotate it, for the previous set of images, and them more flats after the next set of images.

Technically you should also take Flat Darks (the same as your light darks but at the same length of time as your flats) but that's a bit much at this time.

[leon]

Thank you gentlemen, i did suspect that this was correct but just needed to make sure.

Leon